1. Field of the Invention
The present invention relates to a wavelength dependence (swept-wavelength loss) measuring system for measuring a swept-wavelength loss of an optical element such as an optical fiber as a Device Under Test(DUT).
2. Description of the Related Art
There have been two types of swept-wavelength loss measuring system combined with a tunable laser source and an optical power meter, i.e. a system with a stepwise sweeping wavelength measuring method and a system with a continuous sweeping wavelength measuring method.
The stepwise sweeping wavelength measuring method is a method by which a wavelength of an outputted light of a tunable laser source is made stepwise varied, the wavelength of the outputted light is ascertained by monitoring the wavelength with a wavelength meter each time the wavelength is stepwise varied, and intensity of the light transmitted through the DUT is measured with an optical power meter.
The continuous sweeping wavelength measuring method is a method by which a wavelength of an outputted light of the tunable laser source is made continuously varied, at the same time, measuring trigger signals are made outputted at a certain fixed interval, and each time the measuring trigger signal is outputted, intensity of the light transmitted through the DUT is measured with the optical power meter.
The measuring trigger signals are outputted at positions determined beforehand at specified intervals on a movable part in a tunable laser mechanism (for example, a movable part of a motor) of the tunable laser source. With the position on the movable part made related to the wavelength of the outputted light beforehand, the position on the movable part can be expressed in terms of the wavelength to allow the positional information of the movable part to be easily outputted as wavelength information.
Moreover, the measuring trigger signal can be also generated with the outputted light from the tunable laser source split and made incident on a wavelength dependent device such as an etalon (cf. JP-A-11-2587).
The continuous sweeping wavelength measuring method, due to a measuring principle thereof different from that of the stepwise sweeping wavelength measuring method, can carry out a significantly faster measurement compared with the measurement with the stepwise sweeping wavelength measuring method to be becoming a dominant method for the swept-wavelength loss measurement.
Next, an explanation will be made about a principle of the swept-wavelength loss measurement with a related continuous sweeping wavelength measuring method by using FIG. 2.
In FIG. 2, reference numeral 1 denotes an example of a swept-wavelength loss measuring system with the continuous sweeping wavelength measuring method. The system 1 includes a tunable laser source 2, an optical power meter 3, a displaying arithmetic operation unit 4 and a displaying unit 5.
The tunable laser source 2 includes a tunable laser section 2—2 and a measuring trigger signal generating section 2-1.
Reference numeral 6 denotes a DUT to which the outputted light from the tunable laser source 2 is inputted, and the light transmitted through the object 6 is outputted to the optical power meter 3.
The tunable laser source 2 has an arrangement in which a wavelength of outputted light of the tunable laser section 2—2 is made continuously varied, and at the same time, a measuring trigger signals are outputted to the optical power meter 3 at fixed intervals.
The outputted light from the tunable laser section 2—2 transmits through the DUT 6 to be given to the optical power meter 3 as light to be measured. At the optical power meter 3, an intensity of the light is measured with timing at which the measuring trigger signal is outputted.
Intensity information of the light measured at the optical power meter 3 is inputted to the displaying arithmetic operation unit 4 together with wavelength information of the outputted light converted at the measuring trigger signal generating section 2-1. Both kinds of information are processed at the displaying arithmetic operation unit 4, by a processed output of which a graph is displayed at the displaying unit 5 about an intensity corresponding to a wavelength of the outputted light.
In FIG. 3, there is shown an example of results of swept-wavelength loss measurement obtained by the related swept-wavelength loss measuring system.
In FIG. 3, a horizontal axis represents a wavelength and a vertical axis represents an intensity for graphically presenting the intensity as being plotted against each measured wavelength.
In the swept-wavelength loss measuring system shown in FIG. 2, the measured results as shown in FIG. 3 were obtained on the assumption that the measuring trigger signals provided for measurement are outputted at fixed wavelength intervals over the whole measuring range.
In the graph shown in FIG. 3, the wavelength is shown on the horizontal axis. The wavelength shown here, however, does not represent a directly measured wavelength value of the outputted light, but a value obtained in terms of a position of a movable part in a tunable laser mechanism (for example, a movable part of a motor) of the tunable laser source with the position on the movable part having been made related to the wavelength of the outputted light beforehand.
Therefore, in a related swept-wavelength loss measuring system, no actual wavelength while being swept is measured. Thus, an accuracy of the wavelength presented on the horizontal axis was not necessarily assured.
Namely, in the related continuous sweeping wavelength measuring method, no real time wavelength measurement was carried out with a wavelength of the outputted light being continuous varied. This certainly presents an unsteady factor of some kind between the measuring trigger signal and the actually outputted wavelength information of the outputted light, which affects the accuracy of the wavelength value obtained in the measuring system.
In order to reduce such an unsteady factor, efforts have long been concentrated on “How to accurately output the measuring trigger signal”.
Nevertheless, however accurately the measuring trigger signal was outputted, incapability of measuring a wavelength in being swept still caused a problem in that the accuracy of the obtained wavelength corresponding to the measuring trigger signal was merely based on an assumption from an accuracy of a wavelength measured when the sweeping was made stopped.
It is an object of the invention to enhance and assure an accuracy of a wavelength obtained in a swept-wavelength loss measuring system, for which, by using a measuring trigger signal in a continuous sweeping wavelength measuring method, an intensity of light transmitted through a DUT is measured by a photodetector such as an optical power meter, and at the same time, a wavelength of an outputted light from a tunable laser source is measured in synchronous with the measuring trigger signal, thereby relating the intensity of the transmitted light to the wavelength of the outputted light being swept.